1. Field of the Invention
The present invention relates to backlight assemblies, and more particularly to backlight assemblies for directly backlighting displays.
2. Description of Related Art
Display devices such as liquid crystal displays (LCDs) are commonly used for displays for many electronic devices such as monitors of office automatic (OA) equipment, and monitors of televisions, all applications where light weight, low power, and a flat panel display are desired. An LCD is essentially a light switching device that does not emit any light on its own and displays information by controlling the state of orientation of the liquid crystal molecules, which in turn controls the transmission of light. LCDs may be divided into three types: reflective, transflective and transmissive. Reflective LCDs use ambient light, and requires no backlighting. However, transmissive and transflective LCDs require backlighting. A backlight or backlights are provided to enhance contrast and to enable visibility of information. The backlighting may be enabled using different lighting technologies, depending on the size and usage of the display.
For example, in most desktop monitors and televisions, backlighting may be provided by a number of linear light sources in the form of cold cathode florescent lamps arranged in parallel, with a back reflector to enhance efficiency, and a light diffusing or diffuser layer to provide more uniform illumination to the liquid crystal display. Examples are disclosed in US 2002/0149713 A1 (=JP 14-311418) and US 2002/0149719 A1. For thinner displays, e.g., those used in a laptop computer, small diameter cold cathode fluorescent lamps are situated at one end or at opposing ends of a light guide, which directs the light towards the liquid crystal device. One example is disclosed in JP 04-84122 A.
FIGS. 7 and 8 illustrate an LCD 10 according to the prior art. An exploded view in FIG. 7 illustrates an LCD panel 12, a backlight assembly 14 and a front cover 16. The LCD panel 12 is connected to substrates 18 via transmission control protocols (TCPs) 20. An exploded view in FIG. 8 illustrates the backlight assembly 14.
Referring to FIG. 8, the backlight assembly 14 includes a light emitting structure 22, a support structure 24, a reflector plate 26, a diffuser panel 28, an optical sheet 30 having a light diffusing feature, and a chassis 32. The light emitting structure 22 includes a plurality of linear light sources in the form of cold cathode fluorescent lamps 34 arranged and connected in parallel between an inverter substrate 36 and a return substrate 38, which are interconnected by a return cable 40. FIG. 9 is a circuit diagram.
As shown in FIG. 9, the inverter substrate 36 has a dc power input port 42 connected to a dc power source such as a rechargeable battery, not shown. The inverter substrate 36 also has a plurality, corresponding in number to the plurality of cold cathode fluorescent lamps 34, of high ac voltage output connectors 44. The inverter substrate 36 further has a control signal input port 46. The inverter substrate 36 has a return port 48. In FIG. 9, the reference numeral 50 indicates a dc voltage from the rechargeable battery, the reference numeral 52 a ground line and the reference numeral 54 various inverter control signals. A plurality, corresponding in number to the plurality of cold cathode fluorescent lamps 34, of inverter circuits, generally indicated at 56, are formed on the inverter substrate 36 as drivers for driving cold cathode florescent lamps 34.
As is well known, starting and operating a cold cathode florescent lamp requires a high alternating current (“ac”) voltage. Typical starting voltage is around 1,000 volts AC, and typical operating voltage is about 600 volts AC. To generate such a high ac voltage from a dc power source such as a rechargeable battery, an inverter circuit includes a dc-to-ac inverter having a step-up transformer. Such inverter circuit is described in U.S. Pat. No. 6,630,797 B2 issued to Qian et al., which has been incorporated herein by reference in its entirety.
Each of the cold cathode fluorescent lamps 34 has a high-voltage end and a low-voltage end. At the high-voltage end, each lamp 34 is connected via a high-voltage cable 58 to one of high ac voltage output connectors 44. The low-voltage ends of the cold cathode fluorescent lamps 34 are interconnected within the return substrate 38 and connected via the return cable 40 to the return port 48 of the inverter substrate 36. The return port 48 is grounded.
In FIG. 9, a relatively large phantom-line drawn rectangle 60 illustrates the module size of LCD 10, while a relatively small phantom-line drawn rectangle 62 illustrates the size of display screen.
In the backlit LCD, in a bright ambient viewing environment, reflections from the display screen may reduce the observed contrast significantly, despite the inherently high transmission contrast, which is currently available. Such effects may be partially offset by increasing the backlight intensity by increased number of cold cathode florescent lamps 34, which may be arranged in parallel within the two-dimensional area of display screen as shown in FIG. 10.
Step-up transformers corresponding in number to the increased number of cold cathode florescent lamps 34 are required. Each of the step-up transformers must be situated in the proximity of the associated one of the cold cathode florescent lamps 34. A transmission loss would otherwise occur to cause a drop in alternating voltage. Besides, interconnecting each of the step-up transformers and one of the cold cathode florescent lamps 34 by extending a high-voltage cable to pass areas between other electrical components might cause serious ill-effect on them. However, the step-up transformers are difficult to arrange near the high-voltage ends of the cold cathode florescent lamps, respectively. This difficulty would grow if bulkier step-up transformers are needed to produce higher starting and operating voltages.
Arranging a plurality of step-up transformers requires accounting for the minimum distance between the adjacent two to avoid undesired interference. This minimum distance may be called “the minimum transformer pitch”. Arranging a plurality of cold cathode florescent lamps in parallel requires accounting for a distance between the adjacent two. This distance between the adjacent two lamps may be called “lamp pitch”. Keeping the lamp pitch held greater than or equal to the minimum transformer pitch poses no problem in arranging the step-up transformers in line in a vertical, with respect to the display screen, direction in which the cold cathode florescent lamps are spaced. Because the length of inverter substrate will not exceed the dimension of display screen measured in the vertical direction. However, there are problems if the lamp pitch is greater than the minimum transformer pitch.
The first problem is that an inverter substrate inevitably exceeds the dimension of display screen measured in the vertical direction to thereby cause an increase in size of the entire backlight assembly. There is growing demand for a LCD having a larger display screen. A backlight assembly for such larger display screen uses longer cold cathode florescent lamps arranged in parallel, thus requiring larger step-up transformers. As the step-up transformers are larger, the inverter substrate exceeds further the vertical dimension of the larger display screen against the design trend of narrowing the area surrounding the display screen.
The second problem is in the brightness variance within the display screen such that the brightness becomes less with increased distance from the high-voltage end of each of the cold cathode florescent lamps arranged in parallel thereby resulting in less uniform output distribution. This brightness variance grows beyond a negligible level if a display area becomes larger.
These problems are posed also in an application where a backlight assembly is required to illuminate two display screens between which the backlight assembly is interposed.
It would to desirable to produce a backlight assembly free from the above-mentioned problems.